Catalytic cracking, notably fluidized catalytic cracking (“FCC”), is a conventional (i.e., well known) process for converting higher average molecular weight, higher boiling hydrocarbons to more valuable, lower average molecular weight, lower boiling hydrocarbons. The products are useful as fuels for transportation, heating, etc. In the process, the conversion step is usually conducted by contacting a hydrocarbon feedstock, e.g., a heavy gas oil, with a moving bed of particulate catalyst in the substantial absence of hydrogen at elevated temperatures.
The FCC process is cyclic and includes, for example, separate zones for catalytic feedstock conversion, steam stripping, and catalyst regeneration. In the cycle, feedstock is blended with the FCC catalyst in a catalytic reactor, typically a riser reactor, for catalytic conversion into products. Lower boiling products are separated from the catalyst in a separator, e.g., a cyclone separator, and deactivated catalyst is conducted to a stripper and contacted with steam to remove entrained hydrocarbons; the latter can be combined with vapors from the cyclone separator, and both can be conducted away from the process. Stripped deactivated catalyst contains a carbonaceous residue, called “coke”. Stripped catalyst recovered from the stripper is conducted to a regenerator, e.g., a fluidized bed regenerator, and contacted with a combusting gas, e.g., air, at elevated temperature to burn off the coke and reactivate the catalyst. Regenerated catalyst is then blended with the feedstock entering the riser, completing the cycle.
In continuous, cyclic operation, exothermic coke combustion in the regenerator provides at least a portion of the heat required to balance the endothermic feedstock cracking in the reactor. However, the presence of coke beyond that necessary for heat balance is undesirable since converting feedstock hydrocarbon into catalyst coke diminishes the quantity of hydrocarbon products obtained from the feedstock. There is therefore a need for catalysts that selectively make a greater quantity of hydrocarbon products but less catalytic coke.
Mesoporous FCC catalysts, such as those described in U.S. Pat. No. 5,221,648 are effective for feedstock conversion into high value hydrocarbon products, such as light olefins. Such catalysts have the desirable property that undesirably high catalyst coke levels are avoided in FCC operation. However, such catalysts contain a mesoporous silica-alumina matrix formed from silica sols that undesirably add to the expense of catalyst production. Moreover, conventional sols are acidic, and, consequently, can undesirably affect catalytic constituents, such as zeolite, during catalyst synthesis. There is therefore a need for improved mesoporous catalysts.